This invention relates to a process for the detoxification of aqueous solutions containing cyanides and/or cyano complexes in the presence of heavy metals selected from the group consisting of Mn, Co, Ni, Cu, Cd and Zn, more particularly copper, by oxidative treatment thereof with peroxide compounds.
Wastewaters containing cyanides and/or cyanohydrins can be detoxified by the process according to U.S. Pat. No. 3,970,554 in which peroxide compounds are added at temperatures of 10.degree. to 80.degree. C. and at pH values of 6 to 12 in the presence of iodide ions as catalyst and, optionally, silver ions. Hydrogen peroxide is said to be the preferred peroxide compound although alkali metal and alkaline earth metal peroxides may be used in the same way. Although perborates, percarbonates and persulfates are also described as effective in this document, comparison of test 10 (Example 2) with test 25 (Example 7) demonstrates the superiority of hydrogen peroxide to sodium perborate because the reaction time required to reduce the cyanide content to below 0.1 mg/ml was three times longer in the case of the perborate.
According to U.S. Pat. No. 3,970,554, the detoxification of cyanides with hydrogen peroxide proceeds unsatisfactorily in the absence of catalysts. On the other hand, the known copper catalysts have the disadvantage that precisely when the last trace of cyanide is to be decomposed, the decomposition of hydrogen peroxide is also intensively catalyzed. Accordingly, an adequate degree of detoxification is often not achieved.
In the course of the reaction, poorly soluble CuCN; is formed from the copper cyano complexes through step-by-step degradation; if the CuCN is completely degraded by an excess of H.sub.2 O.sub.2, the copper tetrammine complex is formed, preventing precipitation of the copper ion hydroxide form. Accordingly, the treated wastewater may also contain an undesirably large residual quantity of Cu ions.
The problems referred to increase in significance particularly when, in the interests of sufficiently rapid detoxification, a large amount of copper is added to the wastewater to be detoxified, so that copper cyano complexes are initially formed, or when the wastewater-because of its origin-already contains a relatively large quantity of copper cyano complexes and, optionally, other heavy metal cyano complexes, such as those of Cd, Ni, Co, Zn. The process according to U.S. Pat. No. 3,970,554 was intended for the detoxification of wastewaters which are free from heavy metal cyanides and/or do not require the addition of copper salts as a catalyst; only the addition of at most 1 mg Ag.sup.+ /1 as catalyst and, hence, the presence of an equivalent quantity of the Ag(CN).sub.2.sup.- complex was regarded as appropriate.
Solids-free or solids-containing wastewaters, for example ore suspensions of the type accumulating in the mining industry, for example in the leaching of gold with cyanide, and in the electroplating industry, contain heavy metal cyano complexes, particularly copper cyano complexes, in significant quantities. Wastewaters such as these cannot be satisfactorily detoxified with hydrogen peroxide and/or the necessary molar ratio of H.sub.2 O.sub.2 to cyanide is so high that economy is totally jeopardized.
According to U.S. Pat. No. 3,510,424, the oxidation of cyanide to cyanate with subsequent hydrolysis of the cyanate can be carried out with peroxo acids or salts thereof instead of hydrogen peroxide. The oxidation reaction proceeds more quickly with peroxo monosulfate, for example, than with H.sub.2 O.sub.2 and can be further accelerated by catalysts, such as copper salts. In general, the cyanide content of aqueous solutions containing heavy metal cyano complexes can be sufficiently reduced by this process. However, the economy of the process using peroxo acids is limited by their much higher market price compared with hydrogen peroxide.
According to EP-A 0 355 417, special oxidation mixtures of hydrogen peroxide and sulfuric acid may be used instead of commercially available peroxomonosulfates for the detoxification of cyanides, so that the quantity of H.sub.2 O.sub.2 used can be reduced. Oxidation mixtures of hydrogen peroxide and phosphoric acid have also been proposed (EP-A-0 398 234). However, the use of mineral acid in areas where cyanide-containing wastewaters accumulate or are detoxified involves the risk that mineral acid can enter the cyanide solution in the event of a malfunction so that hydrogen cyanide escapes and environmental safety is put at risk. In addition, the production of the oxidation mixtures mentioned requires a dissolving station, and optionally, a cooling system.
Finally, the quantity of hydrogen peroxide used in cyanide detoxification can also be reduced in accordance with EP-A-0 355 418, although a special precipitant for heavy metals, namely trimercapto-s-triazine, does have to be used for this purpose.
It is known from practice that the detoxification of cyanide-containing wastewaters with peroxide compounds proceeds unsatisfactorily and/or leads to an extremely high consumption of peroxide compounds particularly when manganese is present in the wastewater.
Although, as explained above, there are various known processes for the detoxifaction of aqueous solutions containing cyanides and/or cyano complexes of toxic heavy metals, there is still a need to broaden the possibilities for detoxification using peroxide compounds to enable various parameters to be taken into account in practice.